Dynamic Image Acquisition Techniques

In order to evaluate cardiac motion and ventricular function, images must be acquired dynamically in time while maintaining a direct correlation with the different phases of the cardiac cycle. This is usually performed by synchronizing image acquisition with the electrocardiogram (ECG) signal

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recorded from electrodes located on the body surface. The ECG signal provides a trigger for the acquisition of an image at a selected moment of the cardiac cycle. Traditionally the beginning of a cardiac cycle is identified as the start of ventricular contraction, which corresponds to the QRS pulse on the ECG. An alternative to the ECG signal is the vascular pulse recorded either mechanically or optically from limb extremities. This signal, referred to as peripheral pulse gating (PPG), is used whenever the electrical signal from the ECG is difficult to obtain or is of insufficient quality due to electrical noise or interference.

Some imaging modalities such as contrast angiography or echocardiography rely on "post-synchronization" techniques in which the acquired images are sorted to match the different cardiac phases recognizable from the corresponding ECG tracing recorded simultaneously with the images. Other techniques such as radionuclide angiograms, MRI, and CT images can be acquired "synchronously" by initiating image acquisition directly by the ECG trigger signal. In most cases several successive heart cycles are acquired and combined to generate a set of "representative" images corresponding to an average heart cycle. The total heart cycle is divided into a fixed number of intervals with a representative image for each interval. A dynamic replay of the sequence of these images in a continuous loop will provide the perception ofthe beating heart using a display mode often referred to as "cine-loop" display.

Cardiac images acquired dynamically allow for quantitative ventricular wall motion and cardiac hemodynamic performance. Global parameters obtained from differences in cardiac volumes between different phases of the cardiac cycle are used to evaluate the cardiac output and the general performance of the heart. Differences in output and filling of different compartments of the heart allow evaluation of valvular insufficiencies and abnormal communication channels between the different heart chambers. Dynamic evaluation of myocardial wall thickening and ventricular wall motion allow identification of regional abnormalities in myocardial contractility. Furthermore, some dynamic images depict the progression of blood or contrast media flowing through the heart cavities and the large vessels. These images will allow for the quantitative evaluation of cardiac output as well as the detection and measurement of abnormal hemodynamic flow patterns such as cardiac shunts, valvular insufficiencies, and turbulent flow secondary to stenotic restrictions.

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